Artificial disc technology has been employed to correct damaged spinal discs for relieving back pain and restoring or maintaining intervertebral spacing while attempting to minimize their constraining effects on the normal biomechanical movement of the spine. Two types of artificial discs have generally been employed: the artificial total disc which is designed to substitute for the entire disc, i.e. the annulus, nucleus and possibly the end plates as well; and the artificial nucleus where only the nucleus is replaced with the annulus and end plates remaining intact. The disc of the present invention is not intended to be limited to one or the other of the above types.
A number of prior artificial disc devices include upper and lower members that are rigidly fixed to the adjacent upper and lower vertebrae. These fixed members sandwich a bearing therebetween along which they can slide to allow for relative movement between the adjacent vertebrae, see, e.g. U.S. Patent Application Publication 2002/0035400. However, devices such as these usually require special surface materials and/or surface treatments that allow for bone ingrowth for fixing the members to the vertebrae. Moreover, these devices have had problems with migration where the intermediate bearing body shifts out from between the vertebrae, and thus generally require more complex shapes to form stops for resisting such disc shifting.
In a relatively early approach, a stainless steel ball was employed in the damaged disc area. The ball approach, while effective to provide a good range of motion, tended to create subsidence problems. Over time, the ball would crush into the end plates as loading was fairly concentrated over a small surface on the ball in engagement with the plates. In other words, since these ball implants were not of a size that enabled the load of the spine to be distributed evenly thereacross, the end plates tended to subside or fall around the ball.
There also has been focus on simply replacing the nucleus with a gelled substance either injected directly in the disc or provided in pouches to attempt to reinflate the annulus and provide for load bearing. However, these approaches are limited in their use to patients who have a substantially undamaged disc annulus.
Accordingly, there is a need for an artificial disc that does not significantly inhibit spine movement while still providing the load bearing and spacer functions akin to that of a normal, healthy spinal disc.